a. Field
The disclosure relates to a connector assembly for joining a tower to a base structure where the axial alignment of the tower to the base structure needs to be adjustable, and to a method of using such a connector to assemble a tower structure.
b. Related Art
Currently offshore wind turbine towers have a main tower with a tubular body, the lower end of which has a flange to which is welded a tubular transition piece that extends downwards from the tubular body for connection to a tubular base of a lower supporting structure. The lower supporting structure may be a single tubular pile driven into relatively shallow water and a transition piece is then attached. The upper end of the pile then provides the tubular base. Alternatively, when the water is deeper, the lower supporting structure may include a framework structure formed from a lattice of structural members supported by a number of seabed piles and terminated at an upper end by a tubular member that forms the tubular base for the turbine tower.
When the main tower is to be connected to the tubular base of the lower supporting structure, the tubular transition piece, which has an inner diameter greater than the outer diameter of the tubular base, is inserted over the tubular base. Typically, the drilled or driven pile is installed with its axis aligned to within 0.5° of vertical.
Wind velocities increase at higher altitudes due to decreased surface aerodynamic drag by land or water surfaces, and so there is the need to maximize the height of the turbine blades. The tallest wind turbines have a main tower more than 100 m in height. At a height of 100 m, a misalignment of 0.5° of the main tower with respect to vertical will result in a tilt of about 90 cm from vertical at the top of the main tower. Such a lean will add to the moment loads to be applied to the foundations of the wind turbine. To compensate for this amount of misalignment, it then becomes necessary to increase the strength of the main tower and lower supporting structure and foundations. Ideally the tower needs to be as close to vertical a possible to ensure even loading on the tower and turbine.
Therefore, in order to permit the axial alignment between the tubular body of the main tower and the tubular base to be adjusted, it is known to provide a clearance gap between the outer tubular transition piece and the inner tubular base. The gap allows a certain degree of movement between the tubular body of the main tower and the tubular base prior to these being fixed in place with respect to each other, and also accommodates the fact such tubular members, which may be around 6 m in diameter, may be slightly oval with a deviation from circularity of up to about ±1.5% and with a tolerance on nominal diameter of about ±1.0%.
The transition piece, which may weigh 200 tonnes, is suspended by a crane, and then the orientation of the transition piece is adjusted until this is sufficiently close to vertical. The orientation and spacing transition from the tubular base is then set by internal pads inside the gap at the top of the pile and then fixed in place with a high quality epoxy grout inserted between the outer wall of the pile and the inner wall of the tubular transition piece. The grout may take about one week to cure.
A number of problems have been noted with this alignment and setting procedure.
It takes time for the grout to set, during which the main transition piece has to be kept carefully in place by the crane, all of which is very expensive. Furthermore the grout interface has in some cases shown a tendency over time to break, up resulting in an insecure foundation and expensive remedial work.
It is therefore an object of the present disclosure to provide a connector assembly for joining a pair of tubular members where the axial alignment of the tubular members needs to be adjustable, and to a method of using such a connector assembly.